Studies are actively underway to introduce the femto base station into a cellular system as typified by wideband code division multiple Access (WCDMA) or long term evolution (LTE). By setting the femto base station in the buildings such as general households or offices where a propagation environment is coarse and covering an area within several meters in radius, it is expected to realize higher-speed wireless transmission within a femto base station setting area.
In an existing cellular system, it is presumed that the urban area uses overall operational frequency bands. For this reason, it is difficult to obtain a frequency band dedicated to the femto base station. Therefore, upon introducing the femto base station, it is effective to share a frequency between the existing macro base station (macro node B (MNB)) and the femto base station. In addition, it is also anticipated that an access limiting function based on closed subscriber group (CSG) where only a user setting the femto base station can communicate by using the femto base station is supported.
If the femto base station is introduced into the existing cellular system under this condition, downlink cross interference from the femto base station to the existing macro terminal or downlink cross interference from the existing macro base station to the femtocell user (that is, femto terminal (home user equipment (HUE)) becomes a problem.
Particularly, in the LTE system, since a high-speed bit-rate transmission is performed using the downlink data channel (PDSCH), the base station performs maximum power transmission in the downlink. Therefore, the interference problem in the downlink of the LTE system is serious. That is, a user of the femto base station deployed in the vicinity of the macro base station suffers significant interference from the macro base station. Meanwhile, a macrocell user located in the vicinity of the femto base station deployed near the cell edge of the macro base station suffers significant interference from the femto base station.
In the downlink of the LTE system, a multiple access scheme such as OFDMA is employed. In the OFDMA system, interference is generated when the frequency resource block (frequency RB) allocated to the macro base station and the frequency RB allocated to the femto base station at least partially overlap. The magnitude of this interference varies depending on relative position between the macro base station and the femto base station.
Patent Literatures 1 and 2 disclose the common use of frequency between the existing macro base station and femto base station described above. Patent Literatures 1 and 2 disclose that when the macro base station and the femto base station share the frequency, the transmission power of the femto base station is fixed and is not controlled. The literatures further disclose that in such a case the macrocell throughput may be significantly deteriorated. Regarding this problem, the following technique has been proposed. That is, assuming a WCDMA system as a 3G mobile communication system, the transmission power of the femto base station is determined in accordance with the CPICH reception power from the macro base station having the largest reception power of the common pilot channel (CPICH) and the path loss that the femto base station of itself desires to obtain (for example, see Patent Literature 1).
Specifically, in the femto base station disclosed in Patent Literature 1, the transmission power is controlled as follows. That is, first, the femto base station measures the reception power of CPICH transmitted from each macro base station and calculates the initial transmission power based on the largest CPICH reception power. Then, the femto base station makes the femto terminal to measure the reception power of the pilot transmitted from the femto base station or the path loss from the femto base station to the femto terminal and to report the measurement result. In addition, the femto base station adjusts the transmission power, by considering the reception power of the CPICH transmitted from the macro base station and the path loss reported from the femto terminal. It is possible to reduce downlink cross interference applied from the femto base station to the macro terminal or downlink cross interference applied from the macro base station to the femto terminal by performing the transmission power control in this manner.